The present invention relates to an MRI-resistant implantable device. The implantable device of the present invention permits satisfactory performance in the presence of the electromagnetic fields emanated during magnetic resonance imaging (MRI) procedures. Patients provided with the present invention can undergo MRI procedures, and gain the benefits therefrom, while maintaining the use of the diagnostic and therapeutic functions of the implantable device.
Implantable devices such as implantable pulse generators (IPGs) and cardioverter/defibrillator/pacemaker (CDPs) are sensitive to a variety of forms of electromagnetic interference (EMI). These devices include sensing and logic systems that respond to low level signals from the heart. Because the sensing systems and conductive elements of these implantable devices are responsive to changes in local electromagnetic fields, they are vulnerable to external sources of severe electromagnetic noise, and in particular to electromagnetic fields emitted during magnetic resonance imaging (MRI) procedures. Therefore, patients with implantable devices are generally advised not to undergo MRI procedures.
With the exception of x-ray procedures, MRI procedures are the most widely applied medical imaging modality. Significant advances occur daily in the MRI field, expanding the potential for an even broader usage. There are primarily three sources of energy that could lead to the malfunction of an implantable device, during an MRI procedure. First, a static magnetic field is generally applied across the entire patient to align proton spins. Static magnetic field strengths up to 7 Tesla for whole body human imaging are now in use for research purposes. The increase in field strength is directly proportional to the acquired signal to noise ratio (SNR) which results in enhanced MRI image resolution. Consequently, there is impetus to increase static field strengths, but with caution for patient safety. These higher field strengths are to be considered in the development of implantable devices.
Second, for image acquisition and determination of spatial coordinates, time-varying gradient magnetic fields of minimal strength are applied in comparison to the static field. The effects of the gradients are seen in their cycling of direction and polarity. With present day pulse sequence design and advances in MRI hardware, it is not uncommon to reach magnetic gradient switching speeds of up to 50 Tesla/sec (this is for clinical procedures being used presently). Additionally, fast imaging techniques such as echo-planar imaging (EPI) and turbo FLASH are in use more frequently in the clinic. Non-invasive magnetic resonance angiography uses rapid techniques almost exclusively on patients with cardiovascular disease. Previous research evaluating the effects of MRI on pacemaker function did not include these fast techniques. Therefore, the use of MRI for clinical evaluation for individuals with implantable cardiac devices may be an issue of even greater significance. Rapid MRI imaging techniques use ultra-fast gradient magnetic fields. The polarities of these fields are switched at very high frequencies. This switching may damage implantable devices or cause them to malfunction.
Lastly, a pulsed RF field is applied for spatial selection of the aligned spins in a specimen during an MRI procedure. FDA regulations relative to the power limits of the RF fields are in terms of a specific absorption rate (SAR), which is generally expressed in units of watts per kilogram. These limits may not consider the effects on implantable devices as the deleterious effects of transmission of RF fields in the MRI system may no longer be the primary concern in their design parameters.
While advancements in techniques used to protect implantable devices from MRI fields have been made, the techniques described mainly concern incorporating additional protective circuitry in the implantable devices or providing alternative modes of operation in response to electromagnetic insult. For example, U.S. Pat. No. 5,217,010 to Tsitlik et al. describes the use of inductive and capacitive filter elements to protect internal circuitry; U.S. Pat. No. 5,968,083 to Ciciarelli et al. describes switching between low and high impedance modes of operation in response to EMI insult; and U.S. Pat. No. 6,188,926 to Vock concerns a control unit for adjusting a cardiac pacing rate of a pacing unit to an interference backup rate when heart activity cannot be sensed due to EMI.
However, the techniques described do not provide a fail-safe system in the case that the protective circuitry or the alternative modes of the implantable device fails to protect the implantable device from malfunction due to exposure to electromagnetic fields. What is needed is a modular backup system that is resistant to electromagnetic insult and can support the basic functionality of the implantable device, so that if the device fails to function for a duration, such as during an MRI procedure, the backup system can provide the necessary assistance functions.
The present invention provides an implantable device that is resistant to electromagnetic interference comprising first and second modules and a non-optical arrangement for communication between the first module and the second module. During a normal operating mode the first module performs physiologic functions and the second module is deactivated. When electromagnetic interference is detected, the second module, which is resistant to EMI insult, is activated and the first module is deactivated to further protect its components from EMI.
The present invention also provides an implantable device used to monitor and maintain at least one physiologic function, which is capable of operating in the presence of damaging electromagnetic interference. The implantable device includes primary and secondary modules, each independently protected from EMI damage via at least one shielding and/or filtering, and a non-electrical communication device for communicating in at least one direction between the primary and the secondary modules. The primary module, in response to input from electrical sensing leads, activates the secondary module in a failsafe mode. In the failsafe mode, the secondary module carries out a physiologic function upon activation and in the presence of electromagnetic interference.
In an advantageous embodiment, the physiologic function performed by the implantable device is a cardiac assist function, and the implantable device is a cardiac assist device.